The need for high resolution imaging radars and high data rate telecommunications has direct implications for the employed antennas. Specifically, modern RF front-ends require ultra-wideband (UWB) performance using low profile antennas for inconspicuous
installation. Other functionalities such as beam steering and multiple input multiple output (MIMO) are highly desired in an effort to create diverse, multi-functioning antenna systems.
To this end, antenna arrays have been successfully used for beam steering and MIMO applications. However, a key limitation is narrow bandwidth and often bulky size (i.e. non-conformal). Also, in the past, arrays were designed to have minimum mutual coupling.
This itself limited their bandwidth to that of their individual antenna elements.
More recently, a novel class of antennas referred to as “tightly coupled phased array” (TCPAs) were shown to exhibit UWB performance while residing on a thin substrate. In contrast to traditional arrays, TCPAs utilize the mutual capacitance between array elements to counteract the ground plane inductance. Typically, TCPAs provide very large bandwidths (up to 5:1) while maintaining small thickness (λ/10 at the lowest operational frequency). It has been shown that TCPAs are a class of metamaterial antennas, and thus inherently provide significant wave slow-down that can be harnessed for miniaturization. This miniaturization can be exploited for bandwidth increase. Specifically, utilization of the wave slow-down resulted in a novel UWB interwoven spiral array (ISPA) that achieved
10:1 bandwidth using λ/23 thickness.
Although the design of tightly coupled arrays is well understood, for a successful implementation several key challenges remain to be addressed. Firstly, finite size arrays suffer from reduced bandwidth due to non-uniform excitation and insufficient mutual coupling.
To alleviate this issue, in this dissertation we propose a novel excitation technique based on the characteristic modes (CM) of the mutual impedance matrix of the array. Unlike uniform excitation, the proposed feeding scheme provides for very low active VSWRs for
all array elements, even the ones at the array’s edges. To further improve the finite array bandwidth we considered termination techniques for the edge elements, including resistive and short-/open-circuit terminations. Comparisons between these techniques are provided in terms of the array’s active VSWR, efficiency, realized gain and radiation patterns. We found that simple short-circuit terminations of the edge elements was the most effective.
A second challenge relates to the feeding of the UWB TCPAs. Specifically, designing an UWB balun/impedance transformer while conforming to stringent space, weight, cost, and power constraints is not trivial. To address these issues, we designed a novel UWB feed with ~ 4:1 bandwidth having an impedance transformation ratio of 50Ω-200Ω (or 4:1) as well.
The above contributions led to the development of a 7x7 tightly coupled dipole array. Measurements showed that the dipole array can achieve very low VSWR with a measured realized gain of approximately 3dBi at 200MHz and 7dBi at 600MHz. The aperture efficiency of the array was numerically estimated at about 90% throughout the 200MHz-600MHz band.